How to repeat:
1. Attach a product to >1 categories
2. Delete one or more categories from step 1
3. See 'null' categories are shown in the Product's Relations
We think, if some category has been deleted, it should be removed from the product Relations completely, rather than showing it as 'null'.
The latter logic might possibly affect something somewhere, that's why we're asking to check it in code ..
Thanks!